This invention relates to a method and apparatus for separating mold-damaged raisins from a batch of raisins in which the moldy raisins are thoroughly admixed with undamaged raisins.
Raisins are most commonly produced by laying bunches of grapes out where they are exposed to the sun. After a specified amount of time thus exposed, the grapes assume the dehydrated form in which they are known as raisins. However, if it rains while the grapes are undergoing the drying process, the resulting moisture promotes the growth of mold on the raisins, particularly those so positioned during the sun-drying process that the moisture can not evaporate quickly. Thus, in the event of rain during the raisin production process, the raisins which are brought in from the drying fields will include a substantial amount of moldy raisins mixed in with the undamaged raisins.
There are primarily two types of raisin mold. The first known as "black" mold, can be washed off of the raisins. The second, and more common type of mold is that known as "nodular" mold. Raisins affected by nodular mold cannot be readily reconditioned, and must be discarded.
To ensure that the raisins which ultimately reach the consumer are uniformly of a wholesome quality, the raisins which are damaged by nodular mold must be separated from the sound raisins prior to packaging. In some instances, this separation process comprises a visual inspection of the raisin batches after the raisins have been subjected to a hot water bath which causes the mold spots to turn white. This is followed by manual selection of the moldy raisins. This method yields adequate results only if there are relatively few bad raisins in a given batch.
In addition to turning white, the hot water bath makes these moldy raisins soft and sticky while leaving the good raisins unaffected and, therefore, hard. Several prior art mechanical sorters use that change in texture as a basis for the sorting. For example, in one device, an auger lifts the raisins from the hot water bath and squeezes the soft moldy raisins through a screen which forms the lower portion of the housing around the auger. The good raisins resist the squeezing and are carried along by the auger. That process is effective on those raisins in the outer portion of the auger conveyor, but the auger does not mix the raisins well so that a substantial portion of the bad raisins do not come adjacent to the screen.
Other mechanical devices to squeeze the soft, moldy raisins also have been used. In one, water pushes the soft raisins through holes in a metal surface over which the raisins travel. In another, rollers disposed over a conveyor pinch all of the raisins between the rollers and the conveyor. The moldy raisins tend to stick to the rollers, while the good raisins do not. These devices are helpful, but are not sufficiently effective where the percentage of bad raisins is high.
A mechanical raisin sorting method disclosed in the patent art is U.S. Pat. No. 2,967,614 to Nury et al. In this method, the raisins are soaked in an aqueous solution of hydrogen peroxide which causes the moldy raisins to assume different optical qualities from the good raisins. The raisins are then dispensed individually onto a conveyor in a spaced relation in a single row. An optical scanner, which is sensitive to the optical differences between the moldy raisins and the sound raisins, scans the raisins individually as they pass by the scanner on the conveyor, and when a moldy raisin is encountered by the scanning device, a jet of air is caused to blow the moldy raisin off of the conveyor. It is evident that such a procedure is inconvenient and time consuming, and the requirement that the raisins be placed on the conveyor in a single row in a spaced relationship would necessitate a relatively complex mechanism.
In summary, the raisin industry has yet to find a totally satisfactory means for removing moldy raisins from the good raisins in the batches brought in from the drying fields. The need for such a method has been most acute where the proportion of nodular mold-damaged raisins to unspoiled raisins is relatively large. Consequently, where a large proportion of the raisins in a batch have been damaged by nodular mold, the common practice has been to discard the entire batch, resulting in much waste of good raisins.